1. Field of the Invention
The present invention relates to an optical scanning apparatus that employs pulse-width modulation, an image forming apparatus having this optical scanner, and a method of controlling drive current.
2. Description of the Related Art
In order to express shades of gray of dots that form an image, an optical scanning apparatus varies the light-emission pulse width of a light beam emitted from a light source (laser). This is referred to generally as pulse-width modulation (PWM). For example, if a dark dot is formed, the optical scanning apparatus lengthens (broadens) the light-emission pulse width by PWM. Conversely, if a light dot is formed, the optical scanning apparatus need only shorten (narrow) the light-emission pulse by PWM. Thus, an advantage of using PWM is that shades of gray of a dot can be expressed without changing the amount of laser light (amount of light emitted).
However, owing to a variation in lens transmittance and a variation in photosensitivity of a photosensitive drum possessed by an image forming apparatus equipped with the optical scanning apparatus, a problem with the optical scanning apparatus is that image quality is not constant from one product to another. For example, consider a product A equipped with a lens having a high transmittance and a product B equipped with a lens having a low transmittance. Assume that the amount of laser light emission is the same in products A and B. In this case, since product A has a high lens transmittance, the amount of laser light that reaches the photosensitive drum is large. Conversely, since product B has a low lens transmittance, the amount of laser light that reaches the photosensitive drum is small. The image finally formed by product A has a density greater than that of the image formed by product B. Thus, even though the amounts of light emit from the lasers are equal, there is the danger that image density will not be constant between the two products depending upon such characteristics as the transmittance of the lenses and photosensitivity of the photosensitive drums. Such variations often depend upon machining precision in the manufacturing process and it is very difficult to eliminate them completely.
In an actual product, therefore, use is made of a method of suppressing variations in image quality by setting the amount of laser light used high or low in accordance with the variations. For example, in the case of a product equipped with a lens having a low transmittance or a product equipped with a photosensitive drum having poor photosensitivity, the amount of laser light need only be set to a large value. Conversely, in the case of a product equipped with a lens having a high transmittance or a product equipped with a photosensitive drum having good photosensitivity, it will suffice to set a small value for the amount of laser light.
It is known that with a light source such as a laser, the pulse width of drive current for realizing a target optical pulse width (the duty ratio of a binary current value supplied by a current source) differs depending upon the amount of laser light emission. For example, it is required that pulse width W1 of drive current for realizing a prescribed light-emission pulse width at a first light amount P1 be made larger than pulse width W2 of drive current at a second light amount P2 which is greater than the first light amount P1. Japanese Patent Laid-Open No. 07-022679 proposes a method of solving the problem of a diminished optical pulse width that accompanies an inadequate amount of light emission by making the pulse width of drive current relatively larger when the amount of light emission is relatively small.
However, the phenomenon of a reduction in light-emission pulse width that accompanies a shortfall in amount of light emission gives rise to another problem, namely a decline in the response speed of the light-emission pulses. That is, the rise time of the light-emission pulses becomes slower than the ideal time. As a consequence, density declines further or a shift in ideal dot formation position occurs.